1. The present invention is related to the field of electrochemistry, particularly electrochemical milling and finishing of metal parts, such as gas turbine airfoils.
2. Electrochemical milling is well known and comprises the removal of material from the surface of a metal piece by the application of an electrical potential in the presence of a conductive electrolyte. The process is particularly well suited to imparting a smooth finish to parts having surface irregularities. Exemplary components adapted to being finished by electrochemical milling are precision forged titanium airfoils used in gas turbines. While such parts are often in the whole forged to nearly their final dimension, it is quite typical that the leading and trailing edges will have a sheared flashing stub characteristic of forgings made in closed parting dies. For use in gas turbines, it is required that the airfoils have a rounded and very smooth edge and therefore electrochemical processes have been applied to this end in finishing forgings and other similar parts.
Johnson, U.S. Pat. No. 3,849,273, describes a method and apparatus for radiusing the edge of an airfoil using a hollow electrode through which electrolyte is flowed toward the edge. The electrode is of an arbitrary area and size. Generally, the electrode presents a planar surface to the workpiece edge, the planar surface being perpendicular to the direction of current and electrolyte flow. When it is desired to produce a constantly varying radius along the blade edge, as is required when the thickness of the airfoil varies along its length, it is said that the distance between the edge and the nearest electrode point should be proportioned to the amount of material to be removed locally from the airfoil edge. Consequently, relatively precise control of the spacing between the electrode and the edge is required; this is made particularly difficult in view of the forces caused by the rapidly flowing electrolyte, as well as complications produced when the airfoil has a great deal of twist and the edge is a continuously varying curve.
Lucas, U.S. Pat. No. 3,970,538, describes another approach to finishing of airfoil edges, wherein a split cylindrical tube captures the edge of the airfoil and functions as the electrode. Thus, the electrode is quite closely positioned to the airfoil and the slot in the body of the tool casing must match the shape of the blade being milled. Further, given the particular shape and proximity of the electrode to the airfoil, it is possible that the current flow and resultant material removal might be unacceptably localized, to the extent that a step is formed in the airfoil surface, away from the edge where the electrochemical action is abruptly terminated by the tool configuration.
From the foregoing, it is evident that it has been previously recognized that the simple connection of an arbitrary electrode in an electrical circuit in proximity to an airfoil edge will not desirably finish the edge. But, the solutions of Johnson and Lucas and like inventions require precision machined electrodes specific for each part, or careful and strongly rigid fixturing to maintain precise location between the electrode and the workpiece. There is a need for readily removing material from airfoil edges by a method which requires less criticality in positioning and which lessens the difficulty of formulating and constructing electrodes.